When the murderous 9/11 Gang of Five finally lands in New York for trial, they’ll face an impenetrable wall of security — with five times the normal number of US marshals flooding lower Manhattan and a ring of marksmen watching their every move, sources said yesterday.

National Guardsmen will be placed at major transportation hubs while the NYPD will close off subway entrances near the federal courthouse and intensify bag searches throughout the system, the sources said.

Marshals will be shipped in from out of town and snipers placed on roofs nearby the courthouse, where a death-penalty trial is likely.

Nearby streets will be shut off to motor vehicles in all directions.

One source described the likely cost of all the extra measures as “tremendous.”

When the Islamist fanatics behind the plot that left 3,000 Americans dead are moved from their cages in Guantanamo Bay to the Big Apple — which could be as soon as a month from now — they will likely arrive by private plane, sources said.

Bound, gagged and blindfolded, mastermind Khalid Sheik Mohammed and his crew will get their first taste of American justice when they land in the middle of the night at Stewart Airport, outside Newburgh.

Under the strictest security, they will then likely be put on a helicopter and delivered to a jail near the scene of their heinous crime.

The trip will mirror what the al Qaeda thug behind the 1993 World Trade Center bombing, Ramzi Yousef — Mohammed’s nephew — went through after he was arrested.

During his journey to New York in 1995, FBI officials lifted Yousef’s blindfold as they passed over the Twin Towers to show him they were still standing.

“They wouldn’t be if I had more money,” Yousef chillingly replied.

Federal prison officials have yet to decide whether they will place al Qaeda animals Mohammed, Ramzi Binalshibh, Ali Abd Al-Aziz Ali, Mustafa Ahmad Al-Hawsawi and Waleed Bin Attash together in isolation at the Metropolitan Correctional Center or split them up, sending some to the federal pen in Brooklyn, the sources said.

Either way, they will be placed in solitary confinement and allowed no contact, aside from their lawyers.

Once the trial grows close, they will all be placed at MCC and possibly moved in and out of the courthouse through underground tunnels.

Despite the massive security detail, former federal prison guard Louis Pepe — who was permanently injured in a 2000 stabbing by Osama bin Laden aide Mamdouh Mahmud Salim — said that it was not enough and that the trial should not be here in the first place.

“It’s going to happen again,” said Pepe, who suffered brain damage and was paralyzed on his right side after Salim stabbed him in the eye with a sharpened toothbrush while awaiting trial for the 1998 African embassy bombings.

“They want to kill people. They want jihad. They want to become martyrs. That’s all they want. That’s what they did to me.”

He said that federal prisoners are still allowed to have toothbrushes and other items that could be turned into weapons.

“They are still stupid,” he said. “They still give them these things.”

Officials also are prisoners to political correctness by allowing Muslim prisoners out of their handcuffs to pray, he said.

“They can’t say anything — they have to pray,” he complained.

Meanwhile, the Obama administration last night said that it is considering shipping an unknown number of Guantanamo detainees to a near-empty federal prison in Illinois, about 150 miles west of Chicago.